The present invention relates generally to an overload management system, including a method and an apparatus, for controlling the operation of electrical equipment in a short-term overload region above the equipment's continuous capability operation curve, without suffering significant loss of equipment life. This overload management system is particularly beneficial for use with equipment having load management capability, including thyristor-controlled power equipment, such as high voltage direct current (HVDC) equipment, motor drives, static volt-ampere-reactive (VAR) compensators (SVC), and thyristor-controlled series compensation (TCSC) systems.
The present invention may be used in combination with a modular order distribution logic firing control system, which accommodates the competing objectives of various system demands in an alternating current (AC) power distribution system. These system demands may include minimizing losses in scheduling control, stabilizing transients, damping subsynchronous resonance (SSR) oscillations, damping direct current (DC) offset, and damping power-swings, as well as more efficiently using the overload capability of electrical equipment in accordance with the present invention.
Power equipment is typically rated in terms of continuous capability limits and short-term overload capability limits. The cost of a particular installation may be minimized by designing the equipment for normal operating conditions, and then using an operational philosophy which intentionally uses this inherent overload capability of the equipment. For example, a TCSC system may be designed for normal line current, even though the line current may exceed the steady-state rating for short durations for a limited number of times over the life span of the TCSC equipment.
One common overload index used by designers specifying equipment is the thirty minute rating. The equipment may be operated during a rare overload event in the thirty minute rating region, for instance, while system operators redispatch power flows over the power system. Another commonly used index is the transient rating region, for instance, on the order of a ten second overload rating. The equipment may be operated in the transient rating region after a major contingency, such as a fault, to delay bypassing operations until the system stabilizes.
For instance, typical TCSC systems are designed with the intent that occasionally the inherent short-term overload capability of the capacitor will be used. However, the recently realized ability of a TCSC system to amplify the current through the capacitor using vernier conduction adds significantly to the complexity of the TCSC system, in comparison with a conventional series capacitor system. Such vernier operation of a TCSC system is shown in the related U.S. Pat. No. 5,202,538, to Larsen et al., referenced in the first paragraph above. Thus, any attempt to operate such a vernier controlled TCSC system must also address this current amplification characteristic.
Other controlled series compensation systems have been proposed, but without including any type of an overload management function. For example, such earlier systems are proposed by N. Christl, et al. in the paper entitled "Advanced Series Compensation (ASC) With Thyristor-Controlled Impedance," CIGRE Paper No. 14/37/38-05 (Paris, 1992); and by A. J. F. Keri, et al., in the article entitled "Improving Transmission System Performance Using Controlled Series Capacitors," CIGRE Paper No. 14/37/38-07 (Paris, 1992).
In the past, other overload management systems have been proposed. For example, one primitive overload management system has been used with HVDC equipment. During serious power system disturbances, this earlier management system operates the HVDC system above its continuous rating for a specified length of time. After this overload operation, the HVDC system may return to a normal rate of operation or, in some cases shutdown is required for a specified recovery period. Unfortunately, during this recovery shutdown period, power must be rerouted around the HVDC equipment. Also, this earlier management system is implemented as a discrete function, with step-wise control. Thus, this earlier system is not capable of tailoring its response to match levels of overload operation required to accurately respond to fluctuating system disturbances.
Indeed, earlier strategies for handling equipment overloads in general have allowed cyclic operation between normal and overload conditions with discrete, step-wise transitions between conditions, and with durations for specific time periods. For example, overload operation at 137% may be allowed for two hours during a 24-hour cycle. This daily cycle is referred to in industry standards as an "assumed load cycle," with equipment designed to withstand certain temperature rises during such normal and overload conditions. These conventional operating strategies are illustrated in various equipment standards, such as those for power transformers.
Thus, a need exists for an improved overload management system for efficiently operating electrical devices in their short term overload regions, which is directed toward overcoming, and not susceptible to, the above limitations and disadvantages.